ELASTO-VISCO-PLASTIC MATERIAL MODELS AND THEIR INDUSTRIAL APPLICATIONS

The main goal of this work is the analysis of the procedure for material characterization aimed at elasto-visco-plastic models and their implementation in FEM codes. This will be done in order to perform stress analysis of structural components under repeated cyclical loads and high temperatures The overall development involves theoretical, numerical and experimental methodologies. The study of state of the art of material models, available in FEM codes, has allowed the evaluation of the limits of stress analysis on components subjected to repeated cyclical loads and high temperature. The initial study has also highlighted the necessity to implement complex material models. Traditionally this lack has been faced through the use of factors of safety, which are developed and refined on the basis of the experience and historical backgrounds. For systems where efficient design is of the utmost importance (for example the minimum weight design of an aircraft structure), it is possible that the traditional factors of safety may be overly conservative, so that optimal efficiency cannot be achieved. The implementation of complex material models allows simulating the real material behaviour and taking into account the non-linear interaction between creep and fatigue for structural analysis and life prediction. The results are more dependable and allow reducing the traditional factors of safety, consequently lead to a significant improvement in the designing of structural components. The process of material characterization has been developed through the following phases: designing the necessary experimental data base, choosing the constitutive laws, describing the iterative procedure for determining the coefficients of material model and the procedure of model validation, performed by comparisons between experimental data and simulated ones. It has been described how a poor experimental data base influences the material characterization process and the limits of an automatic procedure. Guidelines have been provided for designing experimental tests able to identify the optimized model parameters. In this way, it will be possible to determine material models with wide range of validity and to allow reducing time and costs of a characterization process. A methodology of structural analysis has been developed through the comparison with procedures of tested validity, in way to allow a correct and safe implementation of elasto-visco-plastic material models. An elasto-visco-plastic analysis on a combustion chamber of aeronautical engine has been performed and component life determined, taking into account the interaction between fatigue and creep.